The present invention relates to a method for digital imaging of a printing form through application of energy, in which a burn-off area is detachably fixed by supporting points in the burn-off area, supporting points being left in place on the printing form by non-imaging of image spots, and in which the burn-off from the burn-off area is detached from the printing form in a cleaning step. Furthermore, the present invention relates to a system for digital imaging of printing forms in such a method, having an energy source, a cleaning unit, a control unit, and an image processing unit with a computing unit.
Use is frequently made in the graphics industry of printing forms which in addition to other method steps are structured or imaged by ablation into ink-receptive (oleophilic) and ink-repellent (oleophobic) sections on a printing area. The imaging of a printing form may take place in an exposure device or directly in a printing unit. The image information is transferred to the printing form as the carrier or master, through the application of energy. In other words, a positive structuring of the printing form takes place. Depending on its characteristics, the material of the printing form is in some cases changed to such a degree that a layer of dust and/or a skin forms on the surface. In a dry offset printing process multi-layer ablation printing forms may be used, whose areas of the top layer are to some extent detached, etched, or loosened, using a laser beam.
For example, the upper layer may be a silicon layer which is disposed upon a metallic or polymer layer (e.g., polyester). The upper layer is ink-repellent, while the layer below it is ink-receptive. By way of a cleaning unit, the burn-off is completely loosened by mechanical means and removed. Such a method for cleaning an imaged printing form of burn-off or imaging residues is described in patent EP 0 887 204 A2, hereby incorporated by reference herein.
In the specific case of imaging of printing forms by ablation in a printing unit of a printing press, complications may arise in the method as described. Provided that only small burn-off areas are detached by the imaging, they can be removed from the printing area by the cleaning unit, through frictional relative movement and vacuum extraction. Removal may, however, be problematical if large areas are imaged, resulting in large burn-off areas being detached. The larger the area that is imaged in one piece, the greater the probability that parts of the burn-off area will become completely detached. If the residual adhesion of the detached burn-off area is too low, a layer of dust and/or a skin forms which may become completely detached in small pieces, free of any control. A controlled cleaning of the printing area and removal of the burn-off is made harder or even impossible thereby. There is a danger that residues of the burn-off will penetrate into the printing unit, and in particular into the inking unit, onto the rubber blanket cylinder, or onto the pressure cylinder, with the result that the quality of the printing which occurs after the imaging may be severely impaired.
In order to make the controlled and complete elimination of the burn-off in the cleaning step possible, burn-off areas can be fixed by supporting points. In a partially detached burn-off area, at least at one location or reference point a dot is created at which the bond between the upper layer and the layer below it is either not loosened by application of energy, or is only partially detached. In other words, a supporting point is created in the form of an un-imaged or omitted dot in the burn-off area.
In normal high-resolution imaging, supporting points are distributed in a uniform grid across the entire printing area. In systems for digital imaging of a printing form with high resolution, typically of the order of 10 micrometers for the individual printing dots, the supporting points are too small to be seen with the naked eye. The impression given by the printed image, or the print quality, is consequently not impaired.
The insertion of supporting points is achieved by modifying the imaging data in the following manner: before imaging, the data is present in digital form, represented as a two-dimensional bit field (bit map, raster). At every position, the bit field has a representation of the information as to whether imaging at a reference point on the printing form corresponding to that position should be undertaken or not. At every position, a bit may be either set or un-set. If at uniform intervals in the two linearly independent directions at certain positions in the bit field individual set bits (on bits, “1”) are replaced by un-set bits (off bits, “0”), supporting points result at the corresponding reference points on the printing form, in the form of small un-imaged areas on a uniform grid. The residual adhesion of the imaged area surrounding the reference point can be increased so far thereby that uncontrolled complete detachment does not occur. It is immediately clear that the number of supporting points has to be kept as low as possible.
Simply transferring the described procedure to low-resolution imaging, however, leads to impairment of the print quality, since individual supporting points may become visible in the printed image.